1. Field of the Invention
The present invention relates to a technical field of a surface mount coil component and the surface mount coil component mounted substrate being substantially rectangular parallelepiped shape in appearance and having an electrode structure formed directly on a core.
2. Description of Related Art
The surface mount coil component has a substantially rectangular parallelepiped shape (chip-shaped) in product appearance as shown in a sectional view in FIG. 4, and includes an electrode structure formed directly on both ends of a pillar-shaped core 21 having a substantially rectangular cross section at least at both ends (typically, a drum type core 1 having flanges 3a, 3b having a substantially rectangular cross section at both ends of a winding core part 2). The surface mount coil component 20 such as a choke coil in which cover resin 5 is molded on a coil part of a winding coil 4 wound on the winding core part 2 is suitable for automatic mounting, and is fixedly mounted to electrode pads 26a, 26b of a mounting substrate 25 (printed circuit board) by reflow soldering.
Focusing on the electrode structure of the surface mount coil component 20 in the related art, mentioned above, it typically has an electrode structure (two-terminal) such that internal electrodes 6a, 6b are formed directly on a pillar-shaped core 21 having a rectangular cross section so as to cover end surfaces 5a, 5b at both ends. The electrodes extend perpendicularly to a core axis and wrap around four peripheral surfaces which surround the end surfaces 5a, 5b (over bottom surfaces, the upper surfaces, and side surfaces of the flanges 3a, 3b of the drum type core 1) like a lid. End portions 4a, 4b of a winding coil 4 are connected to the internal electrodes, and cover resin 5 is molded, then, intermediate electrodes 7a, 7b are formed, and finally external electrodes 8a, 8b of plating conductive films are formed so as to cover the entire intermediate electrodes 7a, 7b. 
Since the electrode structure of this configuration includes the external electrodes 8a, 8b extending widely from the end surfaces 5a, 5b to the four peripheral surfaces adjacent thereto, respectively, this structure is referred to as five-surface electrode structure, hereinafter.
The surface mount coil component 20 is soldered at bottom side portions (one surface) of the external electrodes 8a, 8b thereof to a mounting substrate 25 by solder 22, and is conductively fixed with built-up solder (solder fillets 23) from the electrode parts 26a, 26b of the mounting substrate 25 formed on end sides or lateral sides of the external electrodes 8a, 8b. 
As a publicly known document relating to an electrode structure of a surface mount coil component, JP-A-2000-306757 discloses a coil component 30 having an electrode structure in which connection reliability is improved by providing lid-shaped internal electrodes 6a, 6b (each including a ground layer 61 and an upper layer 62) formed so as to cover four peripheral surfaces and end surfaces of square-shaped flanges 3a, 3b at both ends of a drum type core 1, connecting end portions 4a, 4b of a winding coil 4 wound on a winding core part 2 to the internal electrodes (upper sides of the flanges 3a, 3b in FIG. 5), molding the cover resin 5, providing intermediate electrodes 7a, 7b on the internal electrode 6 except for parts (non-formation parts 6e of the intermediate electrodes) on the end sides thereof, and then providing external electrodes 8a, 8b from the surfaces of the intermediate electrodes 7a, 7b to surfaces of the non-formation parts 6e of the intermediate electrodes over five surfaces.
Also, as another publicly known document relating to a coil component of one-surface electrode structure in which the external electrode is formed substantially on one surface (a surface of a side to be soldered to the mounting substrate), JP-A-58-124213 discloses a surface mount coil component 40 as shown in a side-view in FIG. 6A, and in a front elevation in FIG. 6B as a “chip-type variable inductance device”. In the third line of an upper right column in page 2 to the fourth line in a lower left column in the same page, a mounting method of the electrode structure to the mounting substrate is described. In other words, the chip-type variable inductance device 40 shown in FIG. 6 includes (external) electrodes 35a, 35b laminated on lower surfaces of insulating layers 34a, 34b provided on bottom sides of flanges 32a, 32b, and both ends of a coil 36 is connected respectively to the (external) electrodes 35a, 35b. Then, in this structure, the (external) electrodes 35a, 35b are to be soldered to predetermined patterns (electrode pads 26a, 26b) on the mounting substrate (printed circuit board) 25.
In the surface mount coil component, the electrode surface area is preferably reduced as much as possible to ensure close adhesion with respect to a mounting substrate (the surfaces on the bottom sides of the flanges) in terms of improvement of quality factor characteristics, which is a ratio between the reactance X and the resistance R. Therefore, securing a large surface area of the external electrodes on the flanges of the drum type core, which is generally employed in the surface mount coil component over five surfaces causes variations in magnetic field in the core interior, and eddy current in an area close to the electrode surfaces, thereby lowering the quality factor characteristics as the inductor.
On the other hand, in the case of the surface mount coil components 20, 30 in the related art described above, in the sight of the electrode structure and the mounting strength, since the cohesion strength between the internal electrodes 6a, 6b formed directly on the core which are generally formed of Ag electrodes (formed by printing silver paste containing glass frit) and the intermediate electrodes 7a, 7b formed generally of resin Ag electrode (formed by printing conductive paste containing hardening type resin and dried) is low, if the surface areas of the external electrodes 8a, 8b are small, electrode peel may occur at an interface between them (in particular, at an interface S near the solder fillets 23 in FIG. 4) when an impact or a load is applied after the substrate is mounted, thereby lowering the mounting strength. In other words, reduction of the electrode surface area for improving the quality factor characteristics (employment of single-surface electrode structure) tends to cause electrode peel because of lower mounting strength.
In this regard, since the electrode structure in the surface mount coil components 20, 30 in the related art employ the electrode structure in which the end surfaces and the upper, lower, left and right four peripheral surfaces of the flanges 3a, 3b of the drum type core 1 are entirely covered by the internal electrodes 6a, 6b, the intermediate electrodes 7a, 7b, and the external electrodes 8a, 8b (five-surface electrode structure), a large cohesion area is secured. Therefore, even though the cohesion strength is low, it is compensated by the size of cohesion area. Consequently, at least the standard mounting strength is achieved.
The electrode structure of the surface mount coil component 40 in the related art is low in cohesion strength with respect to the external electrodes, and hence improvement of the cohesion strength is desired.
Therefore, it will be important to reduce the electrode surface area to improve the quality factor characteristics of the surface mount coil, and simultaneously to increase the cohesion strength with respect to the external electrodes.